Expandable stent

ABSTRACT

An expandable stent comprising a tubular body made up of a plurality of separated tubular elements ( 1 ) arranged along a common longitudinal axis. Each tubular element ( 1 ) comprises a plurality of rhombic-shaped closed cell elements ( 2 ) joined by circumferentially extending linking members ( 3 ). The closed cell elements ( 2 ) are expandable to allow the tubular elements, and hence the stent itself, to expand. In the direction of the longitudinal axis of the stent, the extremities of each of the closed cell elements has an enlarged loop ( 30 ) with waisted portions ( 33 ) which allow the tubular elements to interlock to create a stable structure, at least when in the unexpanded condition.

[0001] This invention relates to an expandable tubular stent forimplantation in the lumen of a body duct in order to ensure a passagetherein.

[0002] Such stents are used mainly in the treatment of blood vesselsexhibiting stenoses, and more generally in the treatment of diseases ofvarious anatomical ducts of the human or animal body, such as, forexample, the urinary ducts, especially the urethra, or the digestiveducts, especially the oesophagus.

[0003] The percutaneous implantation of an expandable tubular stent in astenotic blood vessel is generally recommended, for example after aconventional angioplasty procedure, for preventing the dilated vesselfrom closing up again spontaneously or for preventing its occlusion bythe formation of a new atheromatous plaque and the possible recurrenceof stenosis.

[0004] A known type of expandable tubular stent consists of an assemblyof radially expandable, tubular elements aligned along a commonlongitudinal axis and successively joined together in pairs byrespective sets of linking members. Such a stent is disclosed, forexample, in international patent application WO 98/58600 in which eachof the tubular elements consists of a strip forming a zigzag corrugationdefining bent extreme portions which are successively connected togetherin pairs in opposite directions by rectilinear intermediate portions. Byvirtue of this zigzag corrugation, the stent is expandable between afirst, unexpanded state, enabling it to be implanted percutaneously bymeans of an insertion device of reduced diameter, and a second, expandedstate, in which the stent makes it possible to ensure a passage in thelumen of the body duct. Stents of this type are also disclosed ininternational patent applications WO 96/26689 and WO 98/20810.

[0005] To install the stent, it is placed in the unexpanded state on anangioplasty balloon catheter. Once in place, the balloon is inflated inorder to cause the stent to expand. Alternatively, the stent may be madefrom a material which has a recovery capacity, so that the stent mayautomatically expand, once in place.

[0006] According to the invention there is provided a stent comprising atubular body made up of a plurality of separate, radially expandable,tubular elements aligned along a common longitudinal axis, wherein atleast some of the tubular elements each comprise a plurality of closedcell elements, each joined to the next by a circumferentially-extendinglinking member.

[0007] It will thus be seen that each tubular element comprises a closedloop consisting of a series of alternating closed cell elements andcircumferential linking members.

[0008] In most known stents, the tubular elements are physically linkedto one another by longitudinally extending linking members. One or moreof such longitudinally extending linking members may link each pair ofadjacent tubular elements. However, there are a number of advantages tobe obtained by not using longitudinally-extending linking members, sothat the stent consists simply of a collection of separate tubularmembers whose alignment along a common axis to form the stent isachieved by other means. Preferably the tubular elements, as well asbeing expandable, are also compressible.

[0009] By “separate” is meant that the tubular elements are not directlyconnected together by longitudinally-extending linking members. The word“separate” does not imply that the elements may not touch and, as willbe explained below, in certain conditions of the stent, the linkingmembers will touch and will indeed link together. In the absence oflongitudinally-extending linking members, the structural integrity ofthe stent is realised by alternative means, such as:

[0010] 1) A tubular member or framework which is not directly joined tothe adjacent tubular elements but over which or within which the tubularelements are positioned in the desired alignment. For example, theballoon which is used to expand the stent can be used to maintain theposition of the tubular members with respect to one another.

[0011] 2) Interlock means which mechanically holds the tubular memberstogether even though they are not directly joined. An example of thiswould be to provide co-operating interlock means on the tubular elementsthemselves.

[0012] In an embodiment of the invention, both these techniques areemployed: the tubular elements are placed over the balloon andinterlocked together so that the stent remains structurally stableduring its often tortuous passage to the treatment site. Upon expansion,the interlocking is released, and the balloon alone then maintains thepositional stability of the stent components. After the balloon has beendeflated, the expanded stent, which has undergone plastic deformation,maintains its expanded shape and thus keeps the vessel being treated atits desired diameter. The expanded vessel applies a reaction force, dueto its elastic nature, against the stent and thus maintains the positionof the individual tubular elements making up the stent with respect toone another.

[0013] In order to allow the stent to expand it is necessary that thetubular elements be radially expandable. For this purpose, each tubularelement is constructed in such a way that it is expandable in thecircumferential direction. This may be achieved by the closed cellconstruction of the invention in which the expansion capabilities of thetubular elements are contained wholly or primarily in the closed cellelements. To avoid out of balance forces during expansion, it ispreferred that the closed cell elements be positioned symmetrically withrespect to the circumferential linking members, but asymmetricarrangements are also possible.

[0014] The tubular elements making up the stent may be all identical, orthey may be different—for example, a stent could be made up of acombination of tubular elements comprising closed cell elements, andtubular elements constructed in some other way, arranged to createparticular desired properties of the stent as a whole.

[0015] The circumferential linking members may simply consist ofrectilinear members extending in the circumferential direction.Alternatively the circumferential linking members may be angled to thecircumferential direction, so long as they have a component in thecircumferential direction so that the adjacent closed cell elements arespaced apart in the circumferential direction. In a further alternative,the circumferential linking members are not rectilinear, but are someother shape to create particular desired characteristics—for example,the circumferential linking members could be such as to provide a degreeof flexibility in the circumferential direction, although the expansioncapabilities of the tubular element will still be primarily due to theclosed cell elements. Preferably, all of the circumferential linkingmembers are the same length in the circumferential direction so that theclosed cell elements are evenly distributed about the circumference ofthe tubular element.

[0016] The circumferential linking members attach to the closed cellelements at respective spaced attachment points, and each closed cellelement is constructed in such a way that it is capable of expandingfrom a first position in which the attachment points are relativelyclose together to a second position in which the attachment points arerelatively further apart. In this way, the circumferential length of thetubular element can be increased from a relatively low value,corresponding to the unexpanded condition of the stent, to a relativelyhigher value, corresponding to the expanded condition of the stent. Inone possible construction, each closed cell element comprises twoindividual members extending between said attachment points, saidmembers being spaced apart in the direction of the longitudinal axis ofthe stent. Thus, one of said members may be said to be the proximalmember, the other the distal member. The proximal and distal members arepreferably symmetrically arranged about a straight line joining the twoattachment points, this line being coaxial around the circumference withthe general direction of the circumferential linking members.

[0017] The proximal and distal members are capable of bending in orderto enable the expansion of the closed cell element from the firstposition to the second position. This may be achieved in various ways.For example, each of the proximal and distal members may be fabricatedfrom a flexible member which is thus able to bend to accommodate therequired movement. Alternatively, each of the proximal and distalmembers is fabricated by a plurality of relatively rigid side membersjoined by hinge members. In the preferred embodiment, each of theproximal and distal members comprises two such side members joinedtogether by a hinge. Preferably the two side members are of equallength, but they do not need to be; however, for a symmetricconstruction the corresponding side members in each of the proximal anddistal members should be of equal length.

[0018] In an embodiment, each closed cell element has a generallyrhombic or diamond shape, comprising four side members of relativelystiff construction, joined by four hinge members corresponding to thecorners of the rhombus. The circumferential linking members attach tothe closed cell element at the location of opposite hinge members. Thus,each circumferential linking member has, at one end, one of the hingemembers of one closed cell element and, at the opposite end, theopposite hinge member of the adjacent closed cell element.

[0019] It is not essential that all the closed cell elements in eachtubular element are the same shape. In an alternative embodiment everyother closed cell element is of rhombic shape, as described above,whilst the closed cell elements in between comprise “double rhombic”elements, each comprising two rhombic shapes, as described above,aligned in the circumferential direction, but joined by a narrow, butnot closed, neck portion.

[0020] Other arrangements of closed cell elements are possible,according to the circumstances.

[0021] The aforesaid interlock means can conveniently be provided byproviding an enlarged portion at each of the hinge members to which thelink members are not attached. The narrowing side members as theyapproach each hinge member, together with the respective enlargedportion, form a narrow or waist portion which can overlap with anenlarged portion from the next adjacent tubular element. Two such waistportions acting together can thus retain an enlarged portion from thenext adjacent tubular element.

[0022] The interlock means do not have to be provided on every closedcell element. It may be adequate to provide them on just a few closedcell elements, but evenly spaced about the circumference, so as to givea balanced attachment between adjacent tubular elements. For thispurpose some of the closed cell elements may extend further in the axialdirection of the stent than the remaining closed cell elements, so thatthese extended portions may interlink with the adjacent tubular element.

[0023] This enlarged portion can be formed as a flexible open cell witha narrowed neck, or can be formed as a relatively rigid block, fromwhich, for example, the two side members may emerge via a respectivenarrowed portion to act as a hinge—in this latter case, the hinge memberactually consists of two separate hinges.

[0024] In current medical practice, it is often the case that, inaddition to its role in providing ongoing support for the vessel wall,the stent is required to act as a means whereby therapeutic agents mayconveniently be applied. Indeed the trauma caused during the angioplastyprocedure may call for localised drug treatment. In addition, drugs maybe used to counteract restenosis, and for other purposes.Conventionally, such therapeutic agents are contained within some formof coating which is applied to the stent so that the drug will bereleased over a period of time. One problem with such an arrangement,however, is that, whereas the drug needs primarily to be applied throughthe wall of the vessel being treated, in practice as much of the drug isreleased into the fluid, e.g. blood, flowing within the vessel as passesthrough the vessel wall. Not only is the drug which is washed awayeffectively wasted, it can also do positive harm elsewhere if, forexample, it enters a sensitive organ such as the heart.

[0025] Thus, in an embodiment of the invention the stent is equippedwith wells opening into its exterior surface—that surface which, whenthe stent is in place, will face the wall of the vessel beingtreated—said wells being suitable to contain therapeutic agent.

[0026] The wells may comprise holes or grooves opening into the exteriorsurface of the stent, and may or may not pass right through the materialof the stent to the interior of the stent. However, if the wells passthrough to the interior of the stent there is clearly a danger of atleast some of the drug being released into the fluid flowing within thevessel. Therefore it is preferred that, in such a case, that end of thewell which opens into the interior of the stent is constructed, forexample by being made narrower, and/or being plugged by a material whichprevents or considerably reduces the tendency of the therapeutic agentto flow therethrough.

[0027] Thus it is preferred that the well is wholly or primarily open tothe exterior surface of the stent so that the therapeutic agent may actdirectly on the wall of the vessel and does not get washed away by thefluid flowing along the vessel being treated.

[0028] The wells may open onto any suitable exterior surface of thestent. For example, the wells may conveniently be formed in the blockswhich form the enlarged portions of the closed cell elements. Forexample, each block could be formed with a well in the form of a hole,which may or may not be a through hole and which opens into that surfaceof the block which forms part of the exterior surface of the stent.Alternatively the wells may be formed as grooves in the side members ofthe closed cell elements, the grooves opening into that surface of theside members which forms part of the exterior surface of the stent. Itwill be understood, however, that the above positions are given just asexamples.

[0029] As mentioned above, the wells contain therapeutic agents whichare intended to be released at a controlled rate against the wall of thevessel being treated. Not all of the wells necessarily will contain thetherapeutic agent, and not all wells need to contain the sametherapeutic agent. It is possible, for example, that the wells ofdifferent tubular elements contain different therapeutic agent, openingup the possibility of providing mixtures of drugs by choosing particulartubular elements carrying particular drugs to make up the stent. Clearlythis is particularly easy with a stent in which the tubular elements areseparate from one another. The therapeutic agents may also be providedin separate layers within the well, with the drug needed first being inthe top layer, and the drugs needed later in lower layers, in correctsequence.

[0030] In addition, it is possible to provide that some of the wellscontain therapeutic agents which have different rates of release. Forexample the drug contained in the wells of those tubular elements at ornear the ends of the stent could be arranged to have a more rapid or aslower release rate than the remainder.

[0031] The therapeutic agents may be provided in any suitable form forretention in the wells, and for sustained release, once installed withinthe vessel. Examples are liquid, gel or powder form.

[0032] In order that the invention may be better understood, severalembodiments thereof will now be described by way of example only andwith reference to the accompanying drawings in which:

[0033]FIG. 1 is a two-dimensional view of the evolute of the surface ofa stent according to a first aspect of the present invention, in its “ascut” condition;

[0034]FIG. 2 is a view corresponding to FIG. 1, but showing just asingle tubular element;

[0035]FIG. 3 is an enlarged view of one of the closed cell elements inthe embodiment of FIG. 1;

[0036]FIGS. 4A and B are side and perspective views of the stent of FIG.1, but in which the number of elements is just three, in its “as cut”condition;

[0037]FIG. 5 is a perspective view of a single tubular element from thestent of FIG. 1;

[0038]FIGS. 6 and 7 are views similar to FIGS. 4A and 4B respectively,but showing the stent in the crimped condition;

[0039]FIGS. 8 and 9 are views similar to FIGS. 4A and 4B respectively,but showing the stent in the expanded condition;

[0040]FIGS. 10 and 11 are views similar to FIG. 4B, but showing twofurther embodiments showing both the first and second aspect of theinvention;

[0041]FIG. 12 is a view similar to FIG. 2 showing a still furtherembodiment of the invention;

[0042]FIGS. 12A, B and C are views on the lines A-A, B-B and C-Crespectively of FIG. 12;

[0043]FIG. 13 is a view similar to that of FIG. 5, but showing theembodiment of FIG. 12;

[0044]FIG. 14 is an enlarged view of part of FIG. 13;

[0045]FIG. 15 is a view similar to FIG. 2 showing a still furtherembodiment of the invention;

[0046]FIGS. 15A and B are views on the lines A-A and B-B respectively ofFIG. 15;

[0047]FIG. 16 is a view similar to that of FIG. 5, but showing theembodiment of FIG. 15;

[0048]FIG. 17 is an enlarged view of part of FIG. 16;

[0049]FIG. 18 is a view similar to FIG. 2 showing a still furtherembodiment of the invention;

[0050]FIG. 18A is a view on the line A-A of FIG. 18;

[0051]FIG. 19 is a view similar to that of FIG. 5, but showing theembodiment of FIG. 18;

[0052]FIG. 20 is a view similar to FIG. 2 showing a still furtherembodiment of the invention;

[0053]FIG. 21 is a view similar to FIG. 5, but showing the embodiment ofFIG. 20,

[0054]FIG. 22 is a view similar to FIG. 2 showing a still furtherembodiment of the invention;

[0055]FIG. 23 is a view similar to FIG. 5, but showing the embodiment ofFIG. 22; and

[0056]FIG. 24 is a view similar to FIG. 4b, but showing the embodimentof FIG. 22.

[0057] Referring firstly to FIGS. 1 and 4, the stent comprises a seriesof radially expandable tubular elements 1 aligned along a commonlongitudinal axis. Both of these Figures show the stent in its “as cut”condition by which is meant the condition in which it comes out of themanufacturing process. FIG. 1 illustrates the stent folded out in twodimensions, illustrated by the X-Y coordinates printed to the side ofthe drawing. In practice the stent is, of course, a three dimensionalobject, as illustrated in elevation and in perspective in FIGS. 4A and4B respectively; thus it is assumed that the ends 12, 13 of each tubularelement in FIG. 1 are in fact joined so that each element forms a closedloop of generally tubular configuration. In this description thelongitudinal direction of the stent is parallel to the X-axisillustrated in FIG. 1, while the circumferential direction of the stentis parallel to the Y-axis in FIG. 1.

[0058] It will be noted that the tubular elements 1 are separate fromone another in the sense that there is no direct physical link betweenthem, keeping the tubular elements 1 in position. Instead alternativemeans are used to maintain the structural integrity of the stent. Thiswill be explained in more detail below.

[0059] In the stent illustrated, all of the tubular elements areidentical in structure and size although, as mentioned above, this neednot necessarily be the case. A single tubular element 1 is shown, in twodimensional form in FIG. 2, and in three dimensional form in FIG. 5.Each tubular element comprises a plurality of closed cell elements 2equally spaced apart by circumferentially extending linking members 3.In the embodiment illustrated each tubular element 1 comprises sixclosed cell elements 2, spaced apart circumferentially by 60°, but othernumbers of closed cell elements are possible, according to thecircumstances.

[0060] A single closed cell element 2 is shown in enlarged detail inFIG. 3. The closed cell element has a generally rhombic or diamond shapedefined by four side members 24 to 27 joined together by respectivehinge members 20 to 23. The circumferential linking members 3 attachedto respective opposite hinge members 21, 23.

[0061] The hinge members 21, 23 are formed by narrowed sections 28, 29where the respective side members 24/27, 25/26 join the respectivelinking member 3. The hinge members 20, 22 are formed as a loop 30having a narrowed opening 31 into the interior 32 of the cell element.This narrowed opening 31 corresponds to a waisted portion 33 whichcooperates in the interlocking of individual tubular elements 1, as willbe explained below. Before the stent is used, it will generally becrimped to the balloon which will carry it to the treatment site andsubsequently expand it. The crimping process involves compressing the“as cut” stent onto the balloon so that it is securely gripped. Duringcompression the diameter of the tubular elements, decreases and this isachieved by a deformation of the closed cell elements 2 in such a way asto tend to close the elements up—i.e. so that the hinge members 21 and23 move towards one another, thus reducing the circumferential length ofthe tubular element 1. During this process the closed cell elements bendat the hinge members 20 to 23 the crimped condition of the stent isillustrated in FIGS. 6 and 7 and since, in effect, the stent is expandedfrom this condition, the crimped condition can also be regarded as theunexpanded condition of the stent.

[0062] It will be noted in FIGS. 6 and 7 that, in the crimped conditionof the stent, the hinge members 20, 22 belonging to adjacent tubularelements are interlocked, thus maintaining the structural integrity ofthe stent as a whole. This interlocking is achieved by the cooperatinginterlocking shapes of the hinge members 20, 22 in which each of theenlarged loops 30 lie between a pair of waisted portions 33 belonging tocircumferentially adjacent closed cell elements 2 belonging to the sametubular element 1. By careful design, the closed cell elements can beconfigured to grip one another to maintain the shape of the stent sothat it is not dislodged or deformed during its often long and tortuouspassage to the treatment site. The longitudinal flexibility of the stentis ensured in the crimped condition by the fact that each loop 30 isallowed to move longitudinally a short but controlled distance towardsthe adjacent linking member 3. Thus, as the stent is bent longitudinallythe loops 30 on one side move slightly, as described, whilst those onthe other side move in the opposite direction. In an alternativeembodiment (not shown) still greater longitudinal flexibility can beachieved by arranging that the elements are interlocked in such a way asto allow the loops to move, in a controlled manner, in eitherlongitudinal direction.

[0063] When the stent reaches the treatment site, and the physician issatisfied as to its correct position, the balloon carrying the stent isexpanded, in the known manner, to expand the stent from its conditionshown in FIGS. 6 and 7 to its dilated condition shown in FIGS. 8 and 9.During this expansion process, the closed cell element 2 deform to afinal shape clearly illustrated in FIG. 8. It will be seen that thehinge members 21, 23 have moved apart in the circumferential direction,thus increasing the circumferential length of each tubular element 1. Atthe same time, the hinge members 20, 22 of adjacent closed cell elements2 move apart in the circumferential direction thus releasing the gripwhich they had previously exerted on the corresponding members ofadjacent tubular elements. The stent however by now is supported bothfrom within and without and so maintains its structural shape, eventhough the interlocking is released. The support from within comes fromthe balloon which is being internally pressurised to expand the stent;the support from without comes from the wall of the vessel beingtreated.

[0064] It will also be noted that, during expansion, the length, in thelongitudinal direction of the stent, of each of the closed cell elements2 reduces and this effect, in a stent with linking members betweenadjacent tubular elements, causes the overall length of the stent toreduce. This reduction in length is undesirable for various reasons, andit will be seen that the use of independent tubular elements 1substantially eliminates this problem.

[0065]FIGS. 10 and 11 show modified versions of the stent of FIG. 1 inwhich the hinge members 20, 23 are modified from the open loop formdescribed previously.

[0066] The stents of FIGS. 10 and 11 differ from that of FIG. 1 in thatthe hinge members 20, 22 comprise a block 34 of material from which theside members 24/27 and 25/26 emerge, via a respective narrowed portionto act as a hinge. Thus, in this case the hinge members 20, 22 eachcomprise a pair of hinges by which the respective side members 24/27 and25/26 are attached to the blocks 34. Preferably these blocks 34 areformed integrally with the remainder of the tubular element, and are ofthe same material.

[0067] The difference between the embodiments of FIGS. 10 and 11 is inthe shape of the blocks 34 which in the case of FIG. 10 is substantiallyrectangular and in the case of FIG. 11 is substantially circular. Inboth cases, each block 34 acts as an enlarged end in a similar manner toloop 30 of the FIG. 1 embodiment, and defines a narrowed waist portionwhere it joins the adjacent side members. The arrangement is thus ableto interlock the individual tubular elements 1 in the same way asdescribed above.

[0068] The advantages of a stent with independent tubular elements overone in which the tubular elements are linked by linking members can besummarised as follows:

[0069] 1) Manufacture is made easier because only a basic tubularelement has to be cut. Any stent length can readily be created by addingthe appropriate number of tubular elements at the commencement of theassembly or crimping process.

[0070] 2) The crimped stent has a high degree of longitudinalflexibility since it is not restrained by the inter-element linkingmembers of known stents.

[0071] 3) The crimped stent has a high degree of longitudinalconformability due to its tubular elements being interlocked at multiplecell locations.

[0072] 4) There is substantially no shortening of the stent duringexpansion because the shortening of each tubular element does not affectthe stent as a whole.

[0073] 5) Once deployed, the stent has a high degree of longitudinalflexibility and of longitudinal and radial conformability due to theabsence of the restraint imposed by inter-element linking members.

[0074] 6) Once deployed the stent has a good vessel repartition andvessel scaffolding, with homogeneous support for the vessel wall—seeparticularly FIG. 8.

[0075]FIGS. 10 and 11 also illustrate the use of wells for containingtherapeutic agent. It will be seen that, in each of FIGS. 10 and 1 theblocks 34 have formed on their exterior surface a well 35 which isintended to act as a reservoir for a therapeutic agent. Each well 35takes the form of a shallow blind hole which opens into the exteriorsurface which, when the stent is deployed faces the wall of the vesselbeing treated.

[0076] Thus, any therapeutic agent contained within the wells 35 actsdirectly on the wall of the vessel, and is not substantially affected bythe flow of fluid within the vessel.

[0077] Although only a single well 35 is formed in each block 34, it ispossible for multiple smaller wells to be formed, perhaps eachcontaining different drugs. Different drugs can be supplied on differenttubular elements, making it easy to create a stent, as needed,containing an appropriate recipe of drugs.

[0078] The holes making up the wells 35 can be formed as through-holes,and plugged from the interior side to create a blind hole.Alternatively, the through hole can be left, and a suitable substancewhich will resist the washing away of the drug contained within the wellcan be deposited at the inner end of the through hole.

[0079] Although the wells 35 are shown as circular holes, it will beunderstood that other shapes are possible, including multi-sided, squareor rectangular. Alternatively, the wells can be formed as grooves orslots opening into the exterior surface of the block 34.

[0080] The wells may additionally or instead of be provided at otherlocations, such as on the side members 24 to 27 of the closed cellelements 2. However, for this purpose, the side members would have to bemade less deformable than they might otherwise be since any deformationof the reservoir during stent crimping or deployment might result indelamination of the reservoir contents, which would be undesirable. Theblocks 34 are seen as attractive since they suffer substantially lessdeformation than other parts of the stent because their bulk, relativeto the remaining components of the stent, is such that they arerelatively stiff.

[0081] FIGS. 12 to 19 illustrate further embodiments similar to that ofFIGS. 10 and 11, showing alternative arrangements of wells.

[0082] In the embodiment shown in FIGS. 12 to 14, two shapes of wellsare shown. Half of the wells 35 have the shape of a short slot 36 whichopens only into the exterior surface of the tubular element; the otherhalf of the wells 35 have the shape of a slot 37 which opens both intothe exterior surface of the tubular element 1, but also info the edge ofthe tubular element 1. Various combinations of these shaped wells can beused.

[0083] The enlarged view of FIG. 14 is of interest in that it clearlyshows the structure of the left-hand hinge member 20. This can be seento comprise two narrowed (i.e. less wide) portions 50,51 where therespective side members 24 and 27 join the block 34.

[0084] In the embodiment of FIGS. 15 to 17, there is again a combinationof different well shapes: a first type of well 35 formed of a short slot38 extending in the circumferential direction of the stent; a secondtype of well 35 formed of a slot 39 which extends right across the block34 in the circumferential direction of the stent, and is open at bothends.

[0085]FIGS. 18 and 19 show an embodiment in which again two differentstyles of well 35 are shown. On the left hand side a block 40 is formedwithin the loop 30 of a hinge member of the type described above inrelation to the embodiment of FIG. 1. The block 40 is formed with a well35 formed as a blind hole, in a similar manner to the wells 35 of theembodiment of FIG. 11.

[0086] On the right hand side a block 41 is formed outside of the loop30 and, once again, is equipped with a well 35 in the form of a blindhole. Since there is room beyond the hinge members 20, 22, the block 41does not interfere with the interlocking of the tubular element 1together during crimping, as described above.

[0087] The advantages of stents incorporating wells, as described above,can be summarised as follows:

[0088] 1) The well can hold drugs without the need for a polymer matrixcoating. The use of wells can eliminate coating delamination duringstent deployment, thus reducing the risk of thrombosis.

[0089] 2) The absence of a polymer matrix coating eliminates anypotential biocompatibility problems arising from their use.

[0090] 3) Once the stent is fully deployed, the outside surface of thestent is pushed against the wall of the vessel being treated; this meansthat the well is open only towards the vessel wall, to enable diffusionof the drugs into the vessel wall. In addition, the drug cannot bewashed out by the flow of fluid in the vessel and so cannot haveundesired effects elsewhere.

[0091] 4) Compared to a thin (0.1-5 micron) drug layer coated on thestent, the reservoir can be loaded with a high dose and long life time.

[0092] 5) The reservoir dimensions (diameter, length, width, depth) canbe readily varied to the particular circumstances such as blood flowdirection and drug release kinetics.

[0093] 6) Each well can contain a single drug and therefore differentdrugs can be individually held in different wells without the danger oftheir reacting with each other.

[0094] FIGS. 20 to 24 show two further embodiments in which the closedcell elements in each tubular element 1 are not all identical, and inwhich the locating means are not provided on every closed cell element.

[0095] Referring to FIGS. 20 and 21, there is shown an embodiment inwhich each tubular element 1 is made up of two different shapes ofclosed cell element which alternate around the tubular element. Thefirst shape of closed cell element, illustrated under reference 50 issimilar to that of the closed cell elements described above withreference to FIG. 3, except that the loops 30 on one side of the rhombicshaped structure are positioned at the end of a pair of extended arms51,52. As a result these “extended” loops 30 protrude, in the axialdirection of the stent, with respect to the remaining parts of thetubular element 1, and are thus able to interlock with the next adjacenttubular element.

[0096] FIGS. 22 to 24 illustrate an embodiment similar to that of FIGS.20 and 21 but in which the extended loops 30 are open at their neck, asdistinct from the arrangement in FIGS. 20 and 21, where each extendedloop 30 takes the form of a closed ring which is attached at the ends ofthe arms 51,52.

[0097] In both embodiments, the closed cell elements between theelements 50 are of different shape to the elements 50. These elements,given the reference 53, each comprise two rhombic-shaped sections 54,55which are joined by a narrow open neck portion 57.

[0098] The joining of adjacent tubular elements is shown in FIG. 24.FIG. 24 actually shows the embodiment of FIGS. 22 and 23, but it will beunderstood that the same interlocking technique can be used for theembodiment of FIGS. 20 and 21. In relation to FIG. 24, it should also benoted that the drawing shows the tubular elements in their expandedstate—i.e. in a state in which they would not ordinarily beinterlocked—see above.

[0099] The aperture 56 formed within the loop 30 in the embodiment ofFIGS. 20 and 21 could be used as a well for containing a therapeuticagent, in the manner described above. For this purpose, the aperture 56may be a through aperture, plugged at its inner end, or may be a blindbore, opening into the outer surface only.

[0100] The stent which has been described is expandable between anunexpanded state (in practice, probably the crimped condition mentionedabove), in which it is able to be guided inside the lumen through a bodyduct, such as a blood vessel, for example, and an expanded state, inwhich the stent, after a uniform expansion, comes into contact with theinner wall of the body duct, defining a passage of approximatelyconstant diameter inside said duct.

[0101] The stent will generally be forcibly expanded mechanically underthe action of a force exerted radially outwards, for example under theeffect of the inflation of a balloon. However, the stent may be of the“auto-expandable” type, i.e. capable of changing by itself from a first,unexpanded condition under stress, enabling it to be guided through thebody duct, to a second, expanded, working condition.

[0102] The stent may be made of any material compatible with the bodyduct and the body fluids with which it may come into contact.

[0103] In the case of an auto-expandable stent, it will be preferable touse a material with a recovery capacity, for example, stainless steel,Phynox® or nitinol.

[0104] In the case of a stent utilising a forced expansion, a materialwith a low elastic recovery capacity may be used to advantage. Examplesare metallic materials such as tungsten, platinum, tantalum, gold, orstainless steel.

[0105] The tubular elements 1 may be manufactured from a hollow tubewith an approximately constant thickness corresponding to the desiredthickness. The shape of the tubular elements may be formed either bylaser cutting followed by electrochemical polishing, or by chemical orelectrochemical treatment.

[0106] The tubular elements may alternatively be manufactured from asheet of approximately constant thickness corresponding to the desiredthickness of the stent. The geometric configuration of the tubularelements can be obtained either by laser cutting followed byelectrochemical polishing, or by chemical or electrochemical treatment.The sheet cut in this way is then rolled up to form a cylinder andwelded to give the desired final structure.

[0107] After assembly of the tubular elements 1 into a stent of thedesired length, the stent can be deployed in a manner known per se. Inthe case of a stent utilising mechanically forced expansion, theinsertion system will preferably comprise a balloon catheter onto whichthe stent will be crimped in the unexpanded state before beingintroduced into an insertion tube for guiding it to the site to betreated.

[0108] The stent of the invention can be intended for both temporary orpermanent placement in the duct or vessel to be treated.

1. A stent comprising a tubular body made up of a plurality of separate,radially expandable, tubular elements aligned along a commonlongitudinal axis, wherein at least some of the tubular elements eachcomprise a plurality of closed cell elements, each joined to the next bya circumferentially-extending linking member.
 2. A stent as claimed inclaim 1 wherein the tubular elements are also compressible.
 3. A stentas claimed in claim 1 further including interlock means for mechanicallyholding the tubular elements together, at least in an unexpandedcondition of the stent.
 4. A stent as claimed in claim 3 in which saidinterlock means are provided by inter-engaging elements provided on saidtubular elements.
 5. A stent as claimed in claim 4 wherein each of saidclosed cell elements is provided with a respective inter-engagingelement which engages a corresponding inter-engaging element on anadjacent tubular element.
 6. A stent as claimed in claim 1 wherein some,but not all, of said closed cell elements are provided with a respectiveinter-engaging element which engages a corresponding inter-engagingelement on an adjacent tubular element.
 7. A stent as claimed in claim 1wherein each closed cell element is expandable in the circumferentialdirection of the tubular element, thus allowing the tubular element toexpand and contract.
 8. A stent as claimed in claim 7 wherein eachclosed cell element is positioned symmetrically with respect to thecircumferential linking members.
 9. A stent as claimed in claim 7wherein each closed cell element comprises two attachment points at eachof which it joins to a respective circumferential linking member, andwherein the closed cell element is such as to be capable of expandingfrom a first position in which the attachment points are relativelyclose together, to a second position in which the attachment points arerelatively further apart.
 10. A stent as claimed in claim 9 wherein,between said attachment points, each closed cell element comprisesproximal and distal members, mutually spaced apart in the direction ofthe longitudinal axis, said proximal and distal members being capable ofbending to accommodate the expansion from the first position to thesecond position.
 11. A stent as claimed in claim 10 wherein the proximaland distal members of each closed cell element are joined together ateach of their circumferentially spaced ends by means of a respectivehinge member.
 12. A stent as claimed in claim 11 wherein each hingemember is attached at one end of a respectivecircumferentially-extending linking members the other end of the linkingmember having attached thereto the opposite hinge member of the nextadjacent closed cell element.
 13. A stent as claimed in claim 10 whereinthe proximal and distal members each comprise a flexible member joiningthe attachment points.
 14. A stent as claimed in claim 10 wherein theproximal and distal members each comprise two or more relatively rigidside members joined by a hinge.
 15. A stent as claimed in claim 14wherein said four side members together form the shape of a rhombus. 16.A stent as claimed in claim 14 wherein each of said side members is ofrectilinear shape.
 17. A stent as claimed in claims 5 or 10 wherein saidinter-engaging elements are each formed by a respective loop formed byeach of said proximal and distal members.
 18. A stent as claimed inclaim 14 wherein the hinge joining each of said two side memberscomprises a loop which forms one of said inter-engaging elements, andwherein the loop joins the adjacent side members by a waisted portionwhich, together with the corresponding waisted portion from the nextadjacent closed cell element in the same tubular element, forms acooperating inter-engaging element.
 19. A stent as claimed in claim 1wherein all of the closed cell elements making up each tubular elementare of the same shape.
 20. A stent as claimed in claim 1 wherein some ofthe closed cell elements making up each tubular element are of adifferent shape to the remainder.
 21. A stent as claimed in claim 1wherein the exterior surface of the tubular body is equipped with wellswhich open onto its exterior surface, said wells being suitable tocontain one or more therapeutic agents.
 22. A stent as claimed in claim21 in which the wells comprise holes or grooves opening into theexterior surface of the stent.
 23. A stent as claimed in claim 22wherein the holes or grooves are blind, i.e. do not pass through thematerial of the stent.
 24. A stent as claimed in claim 22 wherein theholes or grooves pass through to the interior of the stent.
 25. A stentas claimed in claim 24 in which the inner end of the hole or groove, isplugged by a material which prevents or considerably reduces the flow oftherapeutic agent therethrough.
 26. A stent as claimed in claim 25wherein said material is, or contains, therapeutic agent.
 27. A stent asclaimed in claim 21 wherein the closed cell elements are formed withblocks on each of which are formed one or more of said wells.
 28. Astent as claimed in claim 21 wherein at least some of said wells containmultiple therapeutic agents arranged in layers so as to release insequence.